Influenza virus strains for use in vaccines change from season to season. For several years, vaccines have typically included two influenza A strains (H1N1 and H3N2) and one influenza B strain. To deal with ‘antigenic drift’, the precise strains used for vaccination change from year to year.
Of more concern than antigenic drift is ‘antigenic shift’, in which the subtype of prevailing influenza A viruses changes from H1N1 and H3N2. In particular, it is expected that the H5 subtype of influenza A virus may become prevalent in the near future. As the human population is immunologically naïve to the new hemagglutinin subtype then this antigenic shift will cause a pandemic outbreak of influenza infections. The characteristics of an influenza strain that give it the potential to cause a pandemic outbreak are: (a) it contains a new hemagglutinin compared to the hemagglutinins in currently-circulating human strains, i.e. one that has not been evident in the human population for over a decade, or has not previously been seen at all in the human population; (b) it is capable of being transmitted horizontally in the human population; and (c) it is pathogenic to humans.
In preparing for an influenza pandemic caused by a H5 strain, it has been proposed to use a pre-pandemic vaccination strategy [1]. Patients are immunized with a current H5 strain (from birds) in the hope that the resulting immunity will be useful when the pandemic occurs, despite any antigenic drift that may occur in the meantime.
In October 2006, Sanofi Pasteur announced results on its candidate pre-pandemic vaccine, which was tested in adjuvanted and un-adjuvanted forms at various antigen doses. In November 2006 Novartis Vaccines announced that it was filing for European regulatory approval for an adjuvanted pre-pandemic vaccine. In June 2007 GlaxoSmithKline announced its intention to donate 50 million doses of H5N1 adjuvanted pre-pandemic vaccine to the WHO and in February 2008 its PREPANDRIX™ product received a positive opinion from EMEA's Committee for Medicinal Products for Human Use. All three products have been based on the H5N1/A/Vietnam/1194/04 strain.